In a conventional fabrication process, deep trenches are etched in a semiconductor substrate, lined with oxide, and filled with a filler material. The resulting deep trenches must then be polished and capped off to prevent electrical shorts. During the conventional fabrication process, a deep trench cap oxidation process, such as a thermal oxidation, is performed, where the semiconductor wafer is placed in a high temperature environment so that at least a portion of the filler material is oxidized and converted into a thermal oxide or cap oxide over the top of the deep trench to provide electrical isolation.
During the deep trench cap oxidation process, however, materials formed in and around the deep trench, including the filler material in the deep trench and field oxide material around the deep trench, experience volume expansion in various directions in the semiconductor substrate, resulting in structural deformation around the deep trench and volume displacement stress in the semiconductor substrate. Moreover, due to the volume expansion and structural deformation around the top edges of the deep trench during the deep trench cap oxidation process, additional layers subsequently deposited on the non-flat surfaces in and over the top edges of the deep trenches cannot be completely removed by using, for example, selective etching. Consequently, residues accumulate on the bottom, sidewalls, and top edges of the deep trenches, resulting in large bulges of deep trench nodules.
Deep trench nodules make inspection of trench defects difficult using optical inspection tools. Also, the volume expansion of filler material and field oxide material in and around the deep trenches, after the deep trench cap oxidation process, push and lift up additional layers above the field oxide regions, and cause thermal and mechanical stresses, such as volume expansion displacement stress, in the semiconductor substrate. The volume expansion displacement stress can lead to cracking of the semiconductor substrate, which can result in high leakage current during the operation of the semiconductor devices formed in the semiconductor substrate.
Thus, there is a need in the art for a deep trench isolation structure and method that can effectively eliminate deep trench nodules, and prevent thermal and mechanical stresses in the semiconductor substrate caused by the conventional deep trench cap oxidation process.